The operation of taking the square root (or any other roots) of a real
number has an interesting history which we will not review here. Two
aspects of number theory that have grown directly out of exploring
square roots are, however, irrational numbers (since the square root of
most integers can be shown to be irrational) and imaginary numbers. The
former will not interest us as we already work over at least the
real numbers which include all rationals and irrationals, positive and
negative. Imaginary numbers, however, are a true extension of the
reals.

Since the product of any two non-negative numbers is non-negative, and
the product of any two negative numbers is similarly non-negative, we
cannot find any real number that, when squared, is a negative number.
This permits us to ``imagine'' a field of numbers where the square root
of a nonzero negative number exists. Such a field cannot be identical
to the reals already discussed above. It must contain the real
numbers, though, in order to be closed under multiplication (as the
square of an ``imaginary'' number is a negative real number, and the
square of that real number is a positive real number).

If we define the unit imaginary number to be:

(3.2)

such that

(3.3)

we can then form the rest of the field by scaling this imaginary unit
through multiplication by a real number (to form the imaginary
axis) and then generating the field of complex numbers by summing
all possible combinations of real and imaginary numbers. Note that the
imaginary axis alone does not form a field or even a
multiplicative group as the product of any two imaginary numbers is
always real, just as is the product of any two real numbers. However,
the product of any real number and an imaginary number is always
imaginary, and closure, identity, inverse and associativity can easily
be demonstrated.

The easiest way to visualize complex numbers is by orienting the real
axis at right angles to the imaginary axis and summing real and
imaginary ``components'' to form all of the complex numbers. There is a
one-to-one mapping between complex numbers and a Euclidean two
dimensional plane as a consequence that is very useful to us as we seek
to understand how this ``imaginary'' generalization works.

We can write an arbitrary complex number as for real
numbers and . As you can easily see, this number appears to be a
point in a (complex) plane. Addition and subtraction of complex numbers
are trivial - add or subtract the real and imaginary components
separately (in a manner directly analogous to vector addition).

Multiplication, however, is a bit odd. Given two complex numbers
and , we have:

(3.4)

so that the real and imaginary parts are

(3.5)

(3.6)

This is quite different from any of the rules we might use to form
the product of two vectors. It also permits us to form the so-called
complex conjugate of any imaginary number, the number that one can
multiply it by to obtain a purely real number that appears to be
the square of the Euclidean length of the real and imaginary components

(3.7)

(3.8)

(3.9)

A quite profound insight into the importance of complex numbers can be
gained by representing a complex number in terms of the plane polar
coordinates of the underlying Euclidian coordinate frame. We can use
the product of a number and its complex conjugate to define
the amplitude
that is the polar distance of the
complex number from the complex origin. The usual polar angle
can then be swept out from the positive real axis to identify the
complex number on the circle of radius . This representation can
then be expressed in trigonometric forms as:

(3.10)

(3.11)

(3.12)

where the final result can be observed any number of ways, for example
by writing out the power series of
for
complex and matching the real and imaginary subseries with
those for the cosine and sine respectively. In this expression

(3.13)

determines the angle in terms of the original ``cartesian''
complex coordinates.

Trigonometric functions are thus seen to be quite naturally expressible
in terms of the exponentials of imaginary numbers. There is a price to
pay for this, however. The representation is no longer single valued in
. In fact, it is quite clear that:

(3.14)

for any integer value of . We usually avoid this problem initially
by requiring
(the ``first leaf'') but as we
shall see, this leads to problems when considering products and roots.

It is quite easy to multiply two complex numbers in this representation:

(3.15)

(3.16)

(3.17)

or the amplitude of the result is the product of the amplitudes
and the phase of the result is the sum of the two phases. Since
may well be larger than even if the two
angles individually are not, to stick to our resolution to keep the
resultant phase in the range we will have to form a suitable
modulus to put it back in range.

Division can easily be represented as multiplication by the inverse of a
complex number:

(3.18)

and it is easy to see that complex numbers are a multiplicative group
and division algebra and we can also see that its multiplication is
commutative.

One last operation of some importance in this text is the formation of
roots of a complex number. It is easy to see that the square root of a
complex number can be written as:

(3.19)

for any integer . We usually insist on finding roots only within the
first ``branch cut'', and return an answer only with a final phase in
the range .

There is a connection here between the branches, leaves, and topology -
there is really only one actual point in the complex plane that
corresponds to ; the rest of the ways to reach that point are
associated with a winding number that tells one how many times
one must circle the origin (and in which direction) to reach it from the
positive real axis.

Thus there are two unique points on the complex plane (on the
principle branch) that are square roots (plus multiple copies with
different winding numbers on other branches). In problems where the
choice doesn't matter we often choose the first one reached
traversing the circle in a counterclockwise direction (so that it has a
positive amplitude). In physics choice often matters for a specific
problem - we will often choose the root based on e.g. the direction we
wish a solution to propagate as it evolves in time.

Pursuing this general idea it is easy to see that
where
is an integer are the points

(3.20)

where as before. Now we will generally have roots in
the principle branch of and will have to perform a cut to select the
one desired while accepting that all of them can work equally well.